revng-revng/share/revng/compile-flags.cfg

####
#### This file is distributed under the MIT License. See LICENSE.md for details.
####

-std=c11

# Assume we don't have a C-runtime on the target (to allow weird signature on
# main, and to turn off warnings about mismatched c stdlib signatures).
-ffreestanding

# Prevent recompilation of decompiled C code to cause UB on overflows
-fno-strict-overflow

# Treat warnings as errors
-Werror

# Enable a lot of warnings
-Wall
-Wextra

# Warnings that could be enabled in the future if we become better behaved, but
# that we leave disabled for now.
# -pedantic-errors

# Disable a bunch of warning that we know we don't support yet.
#
# For every warning added to this list we an explanation on why it's needed must
# be provided so that we can try and re-enable the warnings in the future.

# All the types from the model are declared with __attribute__((packed)), which
# means that taking the address of any field may result in an unaligned pointer
# value. We just don't care about that, and we can't avoid that, so we are not
# very likely to ever be able to re-enable this warning.
-Wno-address-of-packed-member

# This is usually emitted because MakeModelGEPPass builds two different
# type-based traversal for two pointer expressions that are then compared, which
# ends up with an expression in C that compares two distinct pointer types.
# Unless MakeModelGEPPass becomes perfect and guarantees not to ever emit this
# kind of situations, this warning can hardly be enabled.
-Wno-compare-distinct-pointer-types

# This happens when the second and third operands of a ternary in C have
# different types.
# Eventually we hope to be able to re-enable this warning, when we start
# handling casts and implicit conversion in a robust way.
-Wno-conditional-type-mismatch

# This is usually emitted because MakeModelGEPPass builds two different
# type-based traversal for two pointer expressions that are then compared, which
# ends up with an expression in C that compares two distinct pointer types.
# Unless MakeModelGEPPass becomes perfect and guarantees not to ever emit this
# kind of situations, this warning can hardly be enabled.
-Wno-incompatible-pointer-types

# The following two are caused by the fact that we don't handle very cleanly
# integer casts and pointer casts yet.
# In principle we should be able to remove these when we handle casts properly,
# but at the same time if we end up using implicit casts and conversions for
# improving conciseness of decompiled code these may be impossible to ever
# disable because they would warn against the implicit casts that we're
# exploiting for concisensess.
-Wno-int-conversion
-Wno-int-to-pointer-cast

# We end up dereferencing null pointers in a few situations due to various LLVM
# optimizations that propagate undef, freeze, or poison values.
# In principle we should investigate all these occurrences carefully because
# they are situations where LLVM's optimizations could be breaking semantics.
# We should try to fix this and re-enable this warning to guard against other
# such cases in the future.
-Wno-null-dereference

# TODO: This is caused by some missing parentheses causing weird order of
# evaluation. In principle this should already be handled by
# OperatorPrecedenceResolutionPass, but apparently there are still a few cases
# that should be investigated before re-enabling this warning.
-Wno-parentheses

# This happens because we end up comparing a pointer-typed value with the 0
# constant instead of NULL.
# We should be able to fix this by emitting NULL directly and re-enable this
# warning.
-Wno-pointer-integer-compare

# This happens in situations like: `uint8_t *string = "some_string".
# The compiler complains because we're assigning a pointer to the unique plain
# `char` type to a variable whose type is `uint8_t`, and `char` and `uint8_t`
# have mismatched signedness.
# It's very unlikely that we will be able to re-enable this warning.
-Wno-pointer-sign

# The following happens when we assign a pointer-typed value to an integer
# value. In principle this is worth trying to improve, since it represents an
# opportunity for our type-inference to be better in inferring the type of the
# variable, which should also be a pointer.
# However, given that at some point we plan to exploit C implicit casts and
# conversions to make the decompiled code more concise, we might end up in a
# situation where we're actively exploiting this and we cannot re-enable the
# warning because it would just warn about the implicit cast we're exploiting to
# make the C code less verbose.
-Wno-pointer-to-int-cast

# This is typically triggered when a function that returns non-void does not
# return a value in all control paths.
# This is typically due to function F1 calling F2 who is noreturn. The problem
# is that F2 may be properly detected as noreturn in the model, but it doesn't
# have an __attribute__((noreturn)) when decompiled to C, which causes the
# warning.
-Wno-return-type

# For some reason we end up comparing integers with different signedness. This
# typically involves generic*_t, which is implicitly unsigned in C, but it's not
# signed nor unsigned in the Model.
# Hopefully when we really take care of implicit casts among integers in C we'll
# be able to enable this warning again.
# However, if we end up making purposeful use of implicit casts and conversion
# in C to reduce verbosity of the decompiled code, we will never be able to
# enable this warning again, because it would end up warning us about implicit
# conversion that we are exploiting on purpose.
-Wno-sign-compare

# This just triggers a very big weird case as of today. We could think of
# re-enabling this warning but we have to deal with that first.
-Wno-tautological-constant-out-of-range-compare

# We happen to have uninitialized local variable that are used before being
# written. They are leftover and should go away, but for now they are still
# common, so this warning should be disabled.
-Wno-uninitialized

# This typically happens in the following 2 scenarios.
# 1) Some LLVM's optimization cuts away some piece of the CFG, because it
# recognizes it as dead code, so the variable is left set but never used,
# because it was only used in the dead removed part.
# 2) We initialize the variable with the return value of a call instruction, but
# then the variable is never used again. This is due to some quirks of how
# variables associated with function calls are created.
#
# Both cases should be handled correctly, so eventually we should be able to
# re-enable this warning.
-Wno-unused-but-set-variable

# The following is typically emitted for typedefs of local stack types for
# functions that end up not using the stack.
-Wno-unused-local-typedefs

# This happens if the argument of a function as specified in the prototype is
# never used, which may be caused by a bug in argument detection but may also be
# a legit unused argument, or even the consequence of a user-provided function
# prototype.
# We don't expect to be able to enable this warning, unless we inspect the body
# of the function when printing the prototype and omit the names of the unused
# parameters.
-Wno-unused-parameter

# The following two tend to be emitted when some optimizations remove some
# branches that are considered dead.
# For now we have disabled them because they happen quite often, but we should
# really look into this and figure out if something is wrong in the lifting
# process or in the optimizations we do, because this should not really happen.
-Wno-unused-value
-Wno-unused-variable

# The following is emitted when an isolated function is detected as noreturn by
# revng (and the _Noreturn attribute is hence emitted in the decompiled C code),
# but the body of the function actually calls another dynamic function that is
# not marked noreturn.
# Typically this happens for wrappers to __libc_start_main.
# We might think of fixing this by just manually curating a list of functions
# that should be noreturn, but this is error-prone.
# Also, for instance, the __libc_start_main function does not have the noreturn
# attribute in the libc, so strictly speaking it would even be wrong to mark it
# noreturn.
-Wno-invalid-noreturn

# The following is emitted when in revng we emit a condition in the form:
# ```
# if (&var_0->_offset_1)
# ```
# The compiler warns because taking a non-null address as a boolean always
# returns true.
# On the one hand, this is still valid C, and coming from assembly it may be
# possible.
# On the other hand, a sane compiler would have optimized that away when
# producing the binary, so this pattern may be a hint that we're doing something
# wrong in revng.
# For now we have blindly disabled it because in our arm tests it currently
# happens and it's not high priority at the moment of this writing.
# In the future it might make sense to turn it on again, and properly
# investigate when it happens in our test, to figure out if we're actually
# breaking semantics in some subtle way in some cases.
-Wno-pointer-bool-conversion